System and method for nox reduction optimization

ABSTRACT

An engine controller determines the cost of operating a combustion engine and the cost of operating an emissions after-treatment device. Accordingly, the engine controller adjusts parameters for operation of the engine and the after-treatment device to ensure cost-effective use of the engine and the after-treatment device while complying with exhaust emissions requirements. In particular, the engine controller receives the price of fuel consumed by the engine and the price of reductant used by the after-treatment device to determine the respective cost of operation. Specifically, the fuel is diesel fuel used in a diesel engine; the reductant is urea use in a urea-based Selective Catalytic Reduction (SCR) system; and the regulated exhaust emissions is nitrogen oxide (NOx) emissions. The engine operating parameters may include cooled exhaust gas recirculation airflow, fuel injection timing, fuel injection pressure, and air-to-fuel ratio. The SCR system operating parameters may include the volume of urea injected.

BACKGROUND OF INVENTION

1. Technical Field

The present system and method relate generally to the reduction ofpollutants from emissions released by automotive engines, and moreparticularly to the optimization of reduction of pollutants in exhaustemissions where parameters for operation of the engine and anafter-treatment device are adjusted according to the cost of operation.

2. Description of the Related Art

Due to very high thermal efficiencies, the diesel engine offers goodfuel economy and low emissions of hydrocarbons (HC) and carbon monoxide(CO). Despite these benefits, more efficient operation of diesel enginesresults in higher emissions of nitrogen oxides, i.e., NO or NO₂, knowncollectively as NOx. In diesel engines, the air-fuel mixture in thecombustion chamber is compressed to an extremely high pressure, causingthe temperature to increase until the fuel's auto-ignition temperatureis reached. The air-to-fuel ratio for diesel engines is much leaner(more air per unit of fuel) than for gasoline engines, and the largeramount of air promotes more complete fuel combustion and better fuelefficiency. As a result, emissions of hydrocarbons and carbon monoxideare lower for diesel engines than for gasoline engines. However, withthe higher pressures and temperatures in the diesel engine, NOxemissions tend to be higher, because the high temperatures cause theoxygen and nitrogen in the intake air to combine as nitrogen oxides.

NOx emissions from diesel engines pose a number of health andenvironmental concerns. Once in the atmosphere, NOx reacts with volatileorganic compounds or hydrocarbons in the presence of sunlight to formozone, leading to smog formation. Ozone is corrosive and contributes tomany pulmonary function problems, for instance.

Due to the damaging effects, governmental agencies have imposedincreasingly stringent restrictions for NOx emissions. Two mechanismscan be implemented to comply with emission control regulations:manipulation of engine operating characteristics and implementation ofafter-treatment control technologies.

In general, manipulating engine operating characteristics to lower NOxemissions can be accomplished by lowering the intake temperature,reducing power output, retarding the injector timing, reducing thecoolant temperature, and/or reducing the combustion temperature.

For example, cooled exhaust gas recirculation (EGR) is well known and isthe method that most engine manufacturers are using to meetenvironmental regulations. When an engine uses EGR, a percentage of theexhaust gases are drawn or forced back into the intake and mixed withthe fresh air and fuel that enters the combustion chamber. The air fromthe EGR lowers the peak flame temperatures inside the combustionchamber. Intake air dilution causes most of the NOx reduction bydecreasing the O₂ concentration in the combustion process. To a smallerdegree, the air also absorbs some heat, further cooling the process.

In addition to EGR, designing electronic controls and improving fuelinjectors to deliver fuel at the best combination of injection pressure,injection timing, and spray location allows the engine to burn fuelefficiently without causing temperature spikes that increase NOxemissions. For instance, controlling the timing of the start ofinjection of fuel into the cylinders impacts emissions as well as fuelefficiency. Advancing the start of injection, so that fuel is injectedwhen the piston is further away from top dead center (TDC), results inhigher in-cylinder pressure and higher fuel efficiency, but also resultsin higher NOx emissions. On the other hand, retarding the start ofinjection delays combustion, but lowers NOx emissions. Due to thedelayed injection, most of the fuel is combusted at lower peaktemperatures, reducing NOx formation.

Engine control modules (ECM's), also known as engine control units(ECU's), control the engine and other functions in the vehicle. ECM'scan receive a variety of inputs to determine how to control the engineand other functions in the vehicle. With regard to NOx reduction, theECM can manipulate the parameters of engine operation, such as EGR andfuel injection.

Reducing NOx by manipulating engine operation generally reduces fuelefficiency. Moreover, the mere manipulation of engine operation may notsufficiently reduce the amount of NOx to mandated levels. As a result,after-treatment systems also need to be implemented. In general,catalysts are used to treat engine exhaust and convert pollutants, suchas carbon monoxide, hydrocarbons, as well as NOx, into harmless gases.In particular, to reduce NOx emissions, diesel engines on automotivevehicles can employ a catalytic system known as a urea-based SelectiveCatalytic Reduction (SCR) system. Fuel efficiency benefits of 3 to 10%can result from using SCR systems to reduce NOx rather than manipulatingengine operation for NOx reduction which negatively impacts fuelefficiency. Urea-based SCR systems can be viewed according to four majorsubsystems: the injection subsystem that introduces urea into theexhaust stream, the urea vaporization and mixing subsystem, the exhaustpipe subsystem, and the catalyst subsystem. Several SCR catalysts areavailable for diesel engines, including platinum, vanadium, and zeolite.

ECM's can also control the operating parameters of catalytic converters,such as urea injection in an SCR system. For instance, the ECM can meterurea solution into the exhaust stream at a rate calculated from analgorithm which estimates the amount of NOx present in the exhauststream as a function of engine operating conditions, e.g. vehicle speedand load.

The diesel vehicle must carry a supply of urea solution for the SCRsystem, typically 32.5% urea in water by weight. The urea solution ispumped from the tank and sprayed through an atomizing nozzle into theexhaust gas stream. Complete mixing of urea with exhaust gases anduniform flow distribution are critical in achieving high NOx reductions.

Urea-based SCR systems use gaseous ammonia to reduce NOx. Duringthermolysis, the heat of the gas breaks the urea (CO(NH₂)₂) down intoammonia (NH₃) and hydrocyanic acid (HCNO). The ammonia and the HCNO thenmeet the SCR catalyst where the ammonia is absorbed and the HCNO isfurther decomposed through hydrolysis into ammonia. When the ammonia isabsorbed, it reacts with the NOx to produce water, oxygen gas (O₂), andnitrogen gas (N₂). The amount of ammonia injected into the exhauststream is a critical operating parameter. The required ratio of ammoniato NOx is typically stoichiometric. The ratio of ammonia to NOx must bemaintained to assure high levels of NOx reduction. However, the SCRsystem can never achieve 100% NOx reduction due to imperfect mixing,etc. In addition, too much ammonia cannot be present. Ammonia that isnot reacted will slip through the SCR catalyst bed and exhaust to theatmosphere. Ammonia slip is a regulated parameter which may not exceed afixed concentration in the SCR exhaust.

Urea-based SCR catalysts can be very effective in reducing the amount ofNOx released into the air and meeting stringent emissions requirements.However, the use of urea-based SCR is met with infrastructure anddistribution considerations. As described above, diesel vehiclesemploying urea-based SCR generally carry a supply of aqueous solution ofurea, so a urea distribution system is required to allow vehicles toreplenish their supplies of urea. The United States currently has noautomotive urea infrastructure. The cost of urea is likely to bevolatile in the U.S. even as the first pieces of an infrastructure areput in place, because the development of the urea infrastructure islikely to be slow.

In areas, such as Europe, where the price of diesel fuel is generallymuch higher than the expected price of urea, the SCR system can use asmuch urea as necessary to reduce NOx and achieve maximum fuel economyduring combustion in the engine, notwithstanding any problems with ureadistribution. In contrast, the use of urea in the U.S. will probably bemore measured, because the price of urea will be closer to the price ofdiesel. Moreover, the problems with urea distribution and pricing arecoupled with fluctuations in diesel fuel prices.

SUMMARY OF THE INVENTION

As discussed previously, reducing the content of NOx in exhaustemissions by controlling aspects of engine operation, such as EGR orfuel injection, generally reduces fuel efficiency, because these methodsattempt to lower the temperature at combustion to prevent the formationof NOx. This is disadvantageous when the price of fuel is very high anda premium is placed on fuel efficiency. On the other hand, reducing NOxemissions by increasing the use of a urea-based SCR system, requiresmore urea, and this is disadvantageous when the price of urea is veryhigh. Because the prior art does not dynamically adjust the use of fueland reductants, such as urea, to achieve cost-effective operation of thevehicle, the present invention is a system and method that determinesthe optimal operating parameters for an engine and an emissionsafter-treatment device according to the cost of operating the engine andthe emissions after-treatment device.

An embodiment of the invention employs a combustion engine whichproduces exhaust emissions after combustion of fuel according to engineoperating parameters, an exhaust after-treatment device which acts onthe exhaust emissions according to after-treatment parameters, and anengine controller, such as an ECM, which controls the engine and theafter-treatment device. The engine controller determines a cost tooperate the engine and a cost to operate the after-treatment device. Theengine controller then adjusts the engine operating parameters and/orthe after-treatment parameters, at least partially based on a comparisonof the cost to operate the engine with the cost to operate theafter-treatment device. The engine controller may also adjust the engineoperating parameters and/or the after-treatment parameters based onemissions requirements which specify limits on parts of the overallsystem exhaust.

The engine controller may receive the price of fuel and the price ofreductant as inputs. Moreover, the engine controller may receive datafrom sensors in the engine and the after-treatment system in order tocalculate fuel consumption and urea consumption. The engine controllercan then determine the costs of operating the engine and theafter-treatment device through an algorithm which combines the priceinputs and the consumption calculations to derive the cost of fuelconsumption and urea consumption.

In an exemplary embodiment, the engine is a diesel engine and theafter-treatment device is a urea-based SCR system using urea as areductant to reduce NOx emissions. When the cost of fuel consumption ishigher than urea consumption, the engine controller changes operatingparameters in favor of using the SCR system to reduce NOx and tomaintain a high combustion temperature for higher fuel efficiency. Whenthe cost of urea consumption is higher than the cost of fuelconsumption, the engine-controller changes operating parameters in favorof using the engine to reduce the use of urea while sacrificing somefuel efficiency. While the present invention may be discussedparticularly in terms of implementing an ECM and a urea-based SCR systemto reduce NOx exhaust emissions, the present invention contemplates anyafter-treatment device for reducing any component of exhaust emissions.The embodiments described here are examples to provide a betterunderstanding of the present invention.

If the cost of operating the engine is less than the cost to operate theafter-treatment device, the engine controller may adjust the engineoperating parameters and/or the after-treatment parameters by retardingthe fuel injector timing, decreasing the air-to-fuel ratio, decreasingthe fuel injection pressure, increasing the cooled exhaust gasrecirculation airflow, and/or decreasing from the reductant injectionvolume. On the other hand, if the cost of operating the engine isgreater than the cost to operate the after-treatment device, the enginecontroller may adjust the engine operating parameters and/orafter-treatment parameters by advancing the fuel injector timing,increasing the air-to-fuel ratio, increasing the fuel injectionpressure, decreasing the cooled exhaust gas recirculation airflow,and/or increasing the reductant injection volume. However, the presentinvention contemplates any means for controlling parameters for theoperation of the engine and the after-treatment device.

In many cases, the engine controller must also ensure that the supply ofreductant, such as urea, is not completely depleted. Thus, in anotherembodiment, the engine controller monitors the level of reductant in thereductant supply and reduces reductant usage when the level falls belowa critical threshold. In yet another embodiment, the engine controllerdetermines an optimal rate of reductant usage, which represents thegreatest rate of reductant consumption that will allow the vehicle totravel a certain number of miles starting with a specific amount ofreductant without depleting the supply. The optimal rate of reductantusage can be calculated from input data such as the number of routemiles to be driven and the starting supply of reductant. Thus, theengine controller can ensure that its output signals to theafter-treatment device do not require the after-treatment device to usemore than this optimal rate of reductant usage.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 provides a chart illustrating how the overall system NOx iscreated according to various characteristics of the engine and aurea-based SCR system.

FIG. 2 provides a chart illustrating an exemplary embodiment with thedata that are input into an ECM and how output signals are directed.

FIG. 3 provides a chart illustrating exemplary output signals from theECM to maximize fuel efficiency when the cost of operating the engine ishigher than the cost of operating the SCR system.

FIG. 4 provides a chart illustrating exemplary output signals from theECM to minimize urea usage when the cost of operating the engine islower than the cost of operating the SCR system.

FIG. 5 provides a chart illustrating another embodiment of the presentinvention which utilizes additional input regarding the urea supply.

FIG. 6 provides a chart illustrating exemplary output signals from theECM to minimize urea usage when the supply of urea usage drops below acritical threshold level.

DETAILED DESCRIPTION OF THE INVENTION

Engine controllers, such as ECM's, currently do not account for themonetary cost of operating the engine and the monetary cost of operatingan after-treatment system. More specifically, price inputs for fuel andreductants, such as urea, are not currently specified for ECMalgorithms. As a result, no ECM's, or the vehicles that use them, areable to dynamically adjust the use of fuel and reductants, such as urea,to achieve cost-effective operation of the vehicle while complying withemissions regulations.

The following presents a detailed description of a system and methodthat determines the optimal operating parameters for an engine and anemissions after-treatment device according to the cost of operating theengine and the after-treatment device. To demonstrate the features ofthe present invention, the present invention is discussed in terms of anexemplary embodiment implementing an ECM to reduce total NOx exhaustemissions from a diesel engine by determining appropriate operatingparameters for engine components and for a urea-based SCR systemaccording to the price of diesel fuel and the price of urea. However,this preferred embodiment is not meant to limit the present invention.

Referring to FIG. 1 of the accompanying drawings, overall system NOx 400represents the amount of total NOx exhaust emissions from the entirevehicle, which must fall at or below mandated environmental regulations.Engine NOx 200 represents the NOx exhaust emissions from the operationof the engine 100. The overall system NOx 400 also represents the NOxexhaust emissions that result after the engine NOx 200 passes throughthe urea-based SCR system 300.

Various characteristics of the engine 100 which can affect the amount ofengine NOx 200 include, but are not limited to, the EGR system 110, theinjection timing 120, the injection pressure 130, and the coolanttemperature 140. These engine attributes are merely representative ofthe different ways that the engine NOx 200 can be controlled and areprovided only as an illustration of how the present invention may beimplemented. Moreover, the engine in the present invention generallycovers all aspects of the vehicle, not just those related to fueldelivery and combustion, that occur before emissions are exhausted tothe after-treatment device, which in turn specifically acts to reducethe pollutants in the emissions.

Various characteristics of the urea-based SCR system 300 which canaffect the level of reduction of NOx in the engine NOx 200 include, butare not limited to, the urea injection volume 310, the catalysttemperature 320, and the age of the catalyst 330. These SCR systemattributes are merely representative of how the operation of the SCRsystem 300 can be influenced and are provided only as an illustration ofhow the present invention may be implemented.

Thus, as summarized in FIG. 1, the operation of engine 100 produces theengine NOx 200, and the amount of engine NOx 200 depends on variouscharacteristics of the engine 100. The engine NOx 200 is then introducedinto the SCR system 300 which reduces the amount of NOx in the engineNOx 200 according to the various characteristics of the SCR system 300.The final amount of NOx emissions is the overall system NOx 400.

As shown in the exemplary embodiment of FIG. 2, an ECM 610 is employedfor the present invention. The ECM 610 can be one or moremicroprocessors and other associated components, such as memory deviceswhich store data and program instructions. The ECM 610 generallyreceives input signals from various sensors throughout the vehicle aswell as possible external input data from end users. The ECM 610 thenreads the program instructions and executes the instructions to performdata monitoring, logging, and control functions in accordance with theinput signals and external input data. The ECM 610 sends control data toan output port which relays output signals to a variety of actuatorscontrolling the engine or the SCR system, generally depicted by theengine controls 800 and the SCR system controls 900. In general, thepresent invention can be implemented with most commercially availableECM's and no changes to the ECM will be required. Although thisexemplary embodiment includes an ECM, any system of controllingoperation of engine components and after-treatment devices according tospecified instructions may be employed to implement the presentinvention.

According to the exemplary embodiment of the present invention, the enduser or some input mechanism transmits the unit price of diesel fuel 500and the unit price of urea 510 as input parameters into the ECM 610through the input device 600. The input device 600 may include, but isnot limited to, a computer, personal digital assistant (PDA), or otherentry device with a data link connected physically, wirelessly, or byany data transmission method, to the ECM 610. Moreover, the input device600 may include an automated system or network which transmits data tothe ECM 610. Automatic updates are particularly advantageous where theunit price of diesel fuel 500 and the unit price of urea 510 may changefrequently. If no input parameters are entered, the ECM can use defaultsettings that reflect the most likely prices for diesel fuel and urea.

After receiving the unit price of diesel fuel 500 and the unit price ofurea 510, the ECM 610 determines whether it is more cost-effective toincrease NOx reduction with the engine 100 or with the SCR system 300.The engine sensor data 700 from the engine 100 and the SCR system sensordata 710 from the SCR system 300 provide additional input for the ECM610 to determine optimal operating parameters and to allow the system tochange the parameters dynamically according to changing conditions. Theengine sensor data 700 provides the ECM 610 with data, such as enginespeed and load, required to calculate current fuel consumption, so thatthe ECM 610 can compute the current cost of fuel consumption using theunit price of diesel fuel 500. In addition, the SCR sensor data 710provides the ECM 610 with data required to calculate current ureaconsumption, such as the amount of engine NOx 200, so that the ECM 610can compute the current cost of urea consumption using the unit price ofurea 510. Moreover, the ECM 610 receives data from a sensor in the SCRsystem outflow that indicates overall system NOx to ensure that theoperating parameters are adjusted in compliance with environmentalregulations. Based on the cost calculations, the ECM 610 then sendsoutput signals to the engine controls 800 and the SCR system controls900 directing how the engine 100 and the SCR system 300 should operateto optimize NOx reduction. As the engine sensor data 700 and the SCRsystem sensor data 710 change, the cost calculations may changerequiring the ECM 610 to adjust its output signals.

If the current cost of fuel consumption is higher than the current costof urea consumption, the ECM 610 will attempt to maximize fuelefficiency by maintaining a high temperature at combustion. For example,as shown in FIG. 3, the ECM 610 can maximize fuel efficiency by reducingthe flow of cooled exhaust air back into the combustion chamber. The ECM610 monitors signals from sensors indicating the RPM of the turbochargerin EGR system 810 and sensors indicating engine speed and directs theEGR system 810 to adjust the airflow to increase fuel efficiency.

In addition, the ECM 610 can send signals to calibrate the fuel system820 to maximize fuel efficiency. The ECM 610 can control the rate offuel delivery and the timing of injection through actuators. The ECM 610can also control the pressure at which the fuel is injected. Advancingthe fuel injection, increasing the pressure of injection, and making theair-fuel mixture leaner can be controlled alone or in combination toeffect an increase in fuel efficiency. An engine speed signal may be anecessary sensor input for the ECM 610 to properly regulate the fuelsystem 820.

Meanwhile, since the higher temperatures during combustion increase theengine NOx 200, the ECM 610 can direct the SCR system injection controls910 to increase the amount of urea injected into the SCR system 300 toreduce overall system NOx 400 and ensure compliance with environmentalregulations.

On the other hand, if the current cost of urea consumption is higherthan the current cost of fuel consumption, the ECM 610 will attempt tominimize the need for urea by lowering the temperature at combustion andreducing the engine NOx 200. For example, as shown in FIG. 4, the ECM610 can minimize the engine NOx 200 by increasing the flow of cooledexhaust air back into the combustion chamber. The ECM 610 monitorssignals from sensors indicating the RPM of the turbocharger in EGRsystem 810 and sensors indicating engine speed and directs the EGRsystem 810 to adjust the airflow to decrease the formation of NOx in thecombustion chamber.

In addition, the ECM 610 can calibrate the fuel system 820 to minimizethe need for urea. The ECM 610 can control the rate of fuel delivery andthe timing of injection through actuators. The ECM 610 can also controlthe pressure at which the fuel is injected. Retarding the fuelinjection, decreasing the pressure of injection, and making the air-fuelmixture less leaner all help to increase fuel efficiency. An enginespeed signal may be a necessary sensor input for the ECM 610 to properlyregulate the fuel system 820.

Since the lower temperatures during combustion minimize the engine NOx200, the ECM 610 can direct the SCR system injection controls 910 toreduce the amount of urea injected into the SCR system 300 since lessurea is needed to comply with environmental regulations. It is alsounderstood, however, that urea usage likely cannot be completelyavoided, since there may be limits to the amount that the engine NOx 200can be reduced.

A sensor may also be required to monitor ammonia slip to make sure thattoo much urea is not being introduced and to ensure compliance withregulations governing ammonia slip.

FIGS. 3 and 4 are only exemplary in nature. Controlling the EGR systemand the fuel system in the manner described above are only examples ofhow to affect the combustion temperature and thereby control the amountof NOx. There are also other ways of controlling the amount of ureaneeded in the SCR system. The examples provided are not intended tolimit the methods by which combustion temperature or urea usage arecontrolled. Moreover, the ECM 610 does not have to adjust all theavailable operating parameters that affect fuel efficiency and NOxemissions. For instance, the ECM 610 may be able to increase fuelefficiency without having to increase urea usage if the SCR sensor data710 indicates that the overall system NOx 400 will remain at or belowmandated limits after the adjustment. Thus, the ECM 610 might only sendsignals to adjust engine controls 800. Similarly, if the overall systemNOx 400 will remain at or below mandated limits, the ECM can sendsignals to the SCR system injection controls 910 to reduce the amount ofurea injected into the SCR system 300 without having to reduce fuelefficiency.

FIG. 5 illustrates an additional embodiment of the present inventionwhere the route miles 520 and the starting supply of urea 530 may alsobe entered via input device 600 into ECM 610. The ECM 610 determines anoptimal rate of urea usage 620 which represents the greatest rate ofurea consumption that will allow the vehicle to travel the route miles520 with the starting supply of urea 530 without completely depletingthe supply. The ECM 610 can then prevent complete depletion of urea byensuring that its output signals to the SCR system do not require theSCR system to use more urea than this optimal rate of urea usage 620.Preventing complete depletion eliminates the need to rely on anunreliable urea distribution infrastructure to refill urea tanks or tomake unscheduled stops to replenish. Moreover, it is likely to be morecost-effective for fleets to utilize their own supplies of urea.

Additionally, the ECM 610 can also receive sensor data regarding thelevel of urea in the tank 720 so that when the amount of available ureareaches a critical level, the ECM 610 minimizes urea consumption inorder to prevent complete depletion, which may cause the engine toderate. If the urea level falls below a critical threshold level, theECM 610 can reduce the use of urea and maintain a certain level of NOxemissions by adjusting the engine operating parameters and as depictedin FIG. 6. For example, the EGR airflow is increased, the fuel injectiontiming is retarded, the air-to-fuel ratio is decreased, and/or the fuelinjection pressure is decreased, while the volume of urea injected bythe SCR system is decreased. The actions illustrated in FIG. 6 canoverride the operating parameters that take the cost of fuel and ureainto account. Indeed, reducing the use of urea according to the level ofthe urea supply or measuring urea usage according to an optimal rate ofurea usage can be implemented without determining the costs of operatingthe engine or the SCR system.

It should be readily understood by those persons skilled in the art thatthe present invention is susceptible of broad utility and application.Many embodiments and adaptations of the present invention other thanthose herein described, as well as many variations, modifications andequivalent arrangements, will be apparent from, or reasonably suggested,by the present invention and the foregoing description thereof, withoutdeparting from the substance or scope of the present invention.Accordingly, while the present invention has been described herein indetail in relation to its preferred embodiment, it is to be understoodthat this disclosure is only illustrative and exemplary of the presentinvention and is made merely for purposes of providing a full andenabling disclosure of the invention. The foregoing disclosure is notintended or to be construed to limit the present invention or otherwiseto exclude any such other embodiments, adaptations, variations,modifications and equivalent arrangements.

What is claimed is:
 1. A system for controlling exhaust emissions from acombustion engine, the system comprising: a combustion engine adapted toproduce a first mixture of exhaust emissions after combustion of fuelaccording one or more engine operating parameters; an exhaustafter-treatment device adapted to convert the first exhaust emissions toa second mixture of exhaust emissions according to one or moreafter-treatment parameters, and adapted to inject a reductant from asupply of the reductant into the first exhaust emissions; and an enginecontroller adapted to control the engine and the after-treatment device,and adapted to determine the supply of the reductant, wherein, accordingto the supply of the reductant, the engine controller at least one ofadjusts the one or more engine operating parameters and adjusts the oneor more after-treatment parameters.
 2. The system for controllingexhaust emissions from a combustion engine according to claim 1, whereinthe one or more engine operating parameters comprise at least one of afuel injector timing, an air-to-fuel ratio, a fuel injection pressure,and a cooled exhaust gas recirculation airflow, and wherein the one ormore after-treatment parameters comprise a reductant injection volume.3. The system for controlling exhaust emissions from a combustion engineaccording to claim 2, wherein if the supply of the reductant falls belowa threshold, the engine controller at least one of adjusts the one ormore engine operating parameters and adjusts the one or moreafter-treatment parameters by at least one of retarding the fuelinjector timing, decreasing the air-to-fuel ratio, decreasing the fuelinjection pressure, increasing the cooled exhaust gas recirculationairflow, and decreasing the reductant injection volume.
 4. The systemfor controlling exhaust emissions from a combustion engine according toclaim 1, wherein the reductant is urea.
 5. A method for controllingexhaust emissions from a combustion engine, the method comprising:determining a supply of a reductant for an exhaust after-treatmentdevice; and at least one of adjusting one or more engine operatingparameters and adjusting one or more after-treatment parametersaccording to the supply of a reductant, wherein the one or more engineoperating parameters determine a first mixture of exhaust emissionsafter combustion of fuel by a combustion engine, wherein the one or moreafter-treatment parameters determine a second mixture of exhaustemissions converted from the first exhaust emissions by the exhaustafter-treatment device, and wherein the exhaust after-treatment deviceinjects the reductant from a supply of the reductant into the firstexhaust emissions.
 6. The method for controlling exhaust emissions froma combustion engine according to claim 5, wherein the one or more engineoperating parameters comprise at least one of a fuel injector timing, anair-to-fuel ratio, a fuel injection pressure, and a cooled exhaust gasrecirculation airflow, and wherein the one or more after-treatmentparameters comprise a reductant injection volume.
 7. The method forcontrolling exhaust emissions from a combustion engine according toclaim 6, wherein if the supply of the reductant falls below a threshold,the step of at least one of adjusting the one or more engine operatingparameters and adjusting the one or more after-treatment parametersincludes at least one of retarding the fuel injector timing, decreasingthe air-to-fuel ratio, decreasing the fuel injection pressure,increasing the cooled exhaust gas recirculation airflow, and decreasingthe reductant injection volume.
 8. The method for controlling exhaustemissions from a combustion engine according to claim 5, wherein thereductant is urea.